In the packaging of certain products, and in particular food products, it is highly desirable to impede, and where economically practical, to prevent the transmission of certain gaseous materials into or out of the closed package. While not all gaseous permeations are harmful, the products of concern are caused to deteriorate in quality, and usually in use life, when exposed to certain ones of the various gases normally present in the air.
Addressing food products in particular as representative of the problem addressed herein, certain of the food products are harmed, either aesthetically, or chemically by transmissions especially of oxygen, water vapor, and the like. A typical problem is encountered in snack packaging. Many dry foods such as snacks are advertised and marketed as having certain properties of crispness and freshness. For consumer satisfaction, it is important that these properties be retained through the intended use life. Crispness is usually controlled by control of the moisture level. Freshness is usually associated with both crispness and flavor. Particularly the flavor property is attributed to oxygenation-type reactions with the product. Specifically oxidation of cooking oils, such as in snack chips, produces a change in flavor usually described as rancid. Thus, in this product area, it is significant to control oxygen and water vapor transmission into the package.
Since most packages of the type contemplated herein are formed by activating heat seals, films commonly used have a heat sealable layer on one surface of the film and a more heat resistant layer on the other surface of the film, the two surface layers may be joined directly, or they may have intervening layers to provide interlayer adhesion, barrier properties and the like. It is known to use biaxially oriented polypropylene or biaxially oriented polyesters as the heat resistant layer. The composition of the heat seal layers may be selected from among the commonly known materials, mainly polyethylenes, ethylene copolymers and derivatives thereof. There have also been known uses of saran and certain grades of polypropylene as the sealant layer. Saran is commonly incorporated into the structure as an oxygen barrier layer and may also serve as the heat seal layer in some cases. The polyolefins, and particularly polyethylene provide a certain degree of protection against infusion of moisture vapor.
Recent improvements in barrier layer provisions have resulted from the use of a layer of metal in the film structure. One form of use of metal is known as metallized, wherein a thin layer of metal is vapor deposited onto a polymeric film substrate. While the metallized layer is very thin, it can serve as an effective barrier to transmission of all gases when applied to an appropriate substrate. Metallization also provides an aesthetically pleasing appearance. In presently used metallized structures, the metallization is deposited onto a substrate of either biaxially oriented polyethylene terephthalate (PET) or biaxially oriented polypropylene (OPP). In one known structure the opposite surface of the PET is coated with saran to provide a heat seal layer. In another known structure, a film of PET and PETG (glycol modified polyethylene terephthalate) is metallized on the surface of the oriented PET layer. In that structure the PETG serves as the sealant layer. In both of the above structures, the metallized surface (and sometimes intervening layers) is then laminated to an abuse resistant layer such as OPP or oriented PET.
While the described structures provide good protection, they both have inherently costly features. In the first structure, an additional processing step is required to apply the saran heat seal layer coating. In the second structure, the temperature of heat sealing PETG is higher than desired, giving a narrow heat seal range and making the formed packages susceptible to distortion when heat sealed, and requiring excessive energy usage by the packager.
In the above structures, the salient desirable features are provided by the metallization and the oriented PET. The metallization provides good barrier against gaseous permeation. Typical oxygen permeation rate is no more than about 0.06 cm..sup.3 /100 in..sup.2 24 hrs, at 73.degree. F., 0% R.H. Typical water vapor transmission rate is no more than 0.05 cm..sup.3 /100 in..sup.2 24 hrs., at 100.degree. F., 90% R.H. The oriented PET provides an excellent, smooth and uniform surface for deposition of the metallizing layer, providing for a uniform coverage and bonding of the metal, and thus uniform barrier protection. The oriented PET also imparts a crisp, crinkly feel to the film such as has been found aesthetically pleasing to consumers. The advantages are achieved, however, at an increased cost, either in making the film or in using it, as iterated above. It would be desirable to achieve the same functional film performance while reducing the cost. To wit, the metallized oriented PET should somehow be incorporated into a film which has improved economics.
Various attempts have been made to advantageously utilize the benefits of molecular orientation of films to achieve certain of the desirable properties. Attempts at simultaneously orienting a plurality of layers in a multiple layer film have met with limited success. A problem encountered in these developments has been that each different polymer has its unique set of rheological properties related to the heating and stretching conditions. Especially where certain layers in a multiple layer structure respond to orientation processes by accepting differing amounts of orientation rearrangement at the molecular level, by virtue of differing responses to the temperature present during the orientation process, additional provisions are desirably made to facilitate effecting the simultaneous orientation of the plurality of layers. Absent these provisions, in previous attempts to orient multiple layer films, adjacent layers have tended to develop undesirable stresses at layer interfaces, and cohesive stresses within the layers themselves. These stresses too often have manifested themselves in poor interlayer adhesion at the layer interfaces, and in cracking or hole development in one or more of the layers.
Mueller U.S. Pat. No. 4,188,443 handles this problem in a 5 layer film by selecting the compositions of the second and fourth layers, and an appropriate processing temperature, such that the second and fourth layers are above their melt temperatures during the orientation process (col. 5 line 43). While this mechanism is successful in relieving the interfacial stresses of the orientation process, only 3 of the 5 layers may be truly molecularly oriented. And selection of material compositions for layers 2 and 4 is limited by the melting temperature requirements. There is no teaching wherein adjacent layers are oriented.
Yamada U.S. Pat. No. 4,261,473 teaches a balanced 3 layer film, as in his EXAMPLE 10, wherein the outer layers are polyethylene terephthalate and the core layer is EVOH. Sheets of this film are preheated for a lengthy 5 minutes, apparently to reach steady state temperature throughout the film thickness, before the film is stretched by drawing it into a "cup" shaped mold. This process is more closely related to conventional thermoforming than to molecular orientation.
Mueller U.S. Pat. No. 4,194,039 teaches a "balanced" 3 layer film (col. 6 line 23) that is a combination of olefins and olefin blends. The film is made by a plurality of extrusion steps and orientation steps. The core layer is "hot stretched" as distinguished from the skin layers which are "biaxially oriented" (Col. 8 lines 53-59).
Bornstein U.S. Pat. No. 4,064,296 teaches an oriented 3 layer film having EVOH as the core layer. However, in Bornstein's film it is "crucial" (col. 4 line 65) that one of the two outer layers be cross-linked i.e. by irradiation.
In U.S. Pat. No. 4,501,797 filed Dec. 1, 1982, and of common assignment herewith, there are disclosed four and five layer unbalanced oriented films. The films disclosed therein all have polypropylene as one surface layer.
U.S. Pat. No. 4,501,798 of common assignment herewith discloses seven layer unbalanced oriented films, which include the substructure /Nylon/EVOH/Nylon/.
It is an object of this invention to provide an economical packaging film having adjacent layers which have been simultaneously molecularly oriented, and wherein one of the simultaneously molecularly oriented layers is polyethylene terephthalate. It is a further object to provide such a film with a sealant layer having a low-to-moderate heat sealing temperature. It is still another object to metallize the uniaxially oriented PET layer surface and laminate an abuse resistant oriented polypropylene layer to the metallized PET layer. Such a film is economical to make, with minimum processing steps. It provides excellent barrier to gaseous transmission, and the metallization provides an aesthetically pleasing appearance. The oriented polypropylene layer may be conventionally reverse printed with desired graphics.